While it is clear that Type Ia supernovae (SNe) are the result of thermonuclear explosions in C/O white dwarfs (WDs), a great deal remains uncertain about the binary companion that facilitates the explosive disruption of the WD. Here, we present a comprehensive analysis of a large, unique data set of 127 SNe Ia with exquisite coverage by the Zwicky Transient Facility (ZTF). High-cadence (six observations per night) ZTF observations allow us to measure the SN rise time and examine its initial evolution. We develop a Bayesian framework to model the early rise as a power law in time, which enables the inclusion of priors in our model. For a volume-limited subset of normal SNe Ia, we find that the mean power-law index is consistent with 2 in the rZTF-band ({alpha}r=2.01+/-0.02), as expected in the expanding fireball model. There are, however, individual SNe that are clearly inconsistent with {alpha}r=2. We estimate a mean rise time of 18.9 days (with a range extending from ~15 to 22d), though this is subject to the adopted prior. We identify an important, previously unknown, bias whereby the rise times for higher-redshift SNe within a flux-limited survey are systematically underestimated. This effect can be partially alleviated if the power-law index is fixed to {alpha}=2, in which case we estimate a mean rise time of 21.7 days (with a range from ~18 to 23d). The sample includes a handful of rare and peculiar SNe Ia. Finally, we conclude with a discussion of lessons learned from the ZTF sample that can eventually be applied to observations from the Vera C. Rubin Observatory.

Cone search capability for table J/ApJ/902/47/table3 (90% credible regions for marginalized model parameters (uninformative prior))